1. Field of the Invention
The present invention relates to a method for the formation of images which visualizes an electrostatic or magnetic latent image, or the like, by use of a toner to form a toner image, which toner image is then transferred onto a transfer paper to thereby obtain a final image.
2. Description of the State of the Art
Heretofore, in a method for the formation of an image, for example, in an electrophotographic method, there comprises a process of forming an electrostatic image on a photoreceptor drum, a developing process of visualizing the electrostatic image to produce a toner image, and a fixing process of fixing the toner image. In the fixing process, the toner image that has been formed in the developing process, although it can, as it is, be fixed onto the support, in most cases, is transferred onto another support, and the transferred toner image is then fixed.
For the fixation of the toner image there are known various methods, among which, particularly, the contact-heat fixing method which uses heat rollers is excellent in its high thermal efficiency with the capability of a rapid fixing, so that it is suitable for the fixation in a high-speed copier. Besides, because a relatively low temperature heat source is used, the power consumption can be reduced, thus enabling the design of the copier to be compact and energy saving. Further, in case a paper stays jammed inside the fixing section, there is no possible danger of catching fire, so that the method is favorable also in this respect.
This method, however, has a problem in that it produces "offset phenomenon," the phenomenon being such that part of the toner of a toner image is transferred during fixation onto the surface of a heat roller, which is then retransferred onto the incoming transfer paper to stain the image thereon. In order to prevent the offset phenomenon there may be effectively used such means that a heat roller is provided adjacently thereto with a cleaning member such as a cleaning roller which is to clean the heat roller by removing the toner attached to the roller.
However, in the case where such a cleaning member is provided, the so-called back-staining phenomenon begins to appear. The back-staining phenomenon is such that when the toner material deposited on the cleaning member is subjected to an excessive amount of heat, the toner material is transferred to a pressure roller being pressed against a heat roller. The toner material on the pressure roller thus stains the back of the support such as transfer paper or the like incoming to the position thereafter, and further the toner material is transferred to the heat roller to cause a stain on the support.
On the other hand, the heat fixing method which uses heat rollers requires warming up time before starting the use of the apparatus. The heat roller used herein is composed usually of a metallic cylinder coated therearound with a layer such as a layer of Teflon resin. Conventionally, a heat cylinder composed of an aluminum cylinder has been used for fixing, whose cylinder wall thickness is 4 mm for technical reasons, so that it requires a long warming-up time, e.g., 90 seconds. On the other hand, an effective way for shortening the warming-up time is to reduce the wall thickness of the heat roller. In order to reduce the thickness of the cylinder wall and yet to make it so durable in the structural strength, material having a high Young's modulus such as carbon steel, chromium steel, stainless steel, or the like, needs to be used. However, in the heat roller cylinder made of such a material, the heat conductivity of the material is so small that the entire material is hardly uniformly heated, and the heat roller cylinder is sometimes excessively heated locally when the heat thereon is not lowered by a transfer paper or the like, and thus causing the back-staining phenonmenon.